First Land-Walking Fish Looks Like It Had 'All-Wheel Drive'

University of Chicago scientist Neal Shubin shows a model of Tiktaalik, the "missing link" fish whose fossils he discovered in 2004.

Chris WalkerMCT/Landov

Originally published on January 14, 2014 6:15 pm

A creature that lived 375 million years ago and is thought to have been the first fish to have made the transition to land sported large pelvic bones in addition to its leg-like front fins, new research shows, suggesting that it was a more efficient walker than previously thought.

Tiktaalik roseae, discovered in 2004 on Ellesmere Island in Nunavit, Canada, is a key transitional fossil that links lobe-finned fishes and tetrapods, the first four-limbed vertebrates at the end of the Devonian period.

Sci-News.com describes Tiktaalik as having had "gills, scales and fins, but also a mobile neck, robust ribcage and primitive lungs. In particular, its large forefins had shoulders, elbows and partial wrists, which allowed it to support itself on ground."

However, when the first fossils of Tiktaalik were made public, reconstructions intentionally omitted the tail section because discoverers Neil Shubin, Ted Daeschler and Farish A. Jenkins Jr. and preparator Fred Mullison hadn't finished separating rock from fossil on all their specimens and didn't have a clear idea on how the rest of the creature looked. (NPR's Joe Palca did a profile of Shubin and the discovery of Tiktaalik back in 2010.)

The discovery outlined in the latest issue of Proceedings of the National Academy of Sciences clears that up. The team found a pelvis and partially preserved hind fin in specimens that suggests Tiktaalik had "all-wheel drive," the study authors say.

"The earliest tetrapods have robust limbs, particularly hind limbs that are enlarged and supported by a number of modifications to the pelvic girdle. In contrast, the closest relatives of tetrapods maintain small and weakly ossified pelvic appendages as compared with the pectorals," the team writes.

The pelvic girdle and fin material newly found in Tiktaalik, however, reveals that while it retained a primitive skeletal architecture, its "walking" structures show it was more tetrapod than fish.

"The paper describes a pelvic girdle that is unusually large for a fish, with surfaces where large muscles could have attached. But Tiktaalik still had fins; it would not have been able to walk like a true tetrapod. It also has a hip joint that is intermediate, oriented in a way that is not quite like those of fish or limbed animals.

"Paleontologists not involved in the work said that the find was significant.

" 'It's what we've all been waiting for,' said Jennifer Clack, curator of vertebrate paleontology at the University of Cambridge's Museum of Zoology in the United Kingdom. 'Until this discovery, we weren't able to see the changes by which the pelvic fins of the fish became much larger and more robust, and gradually turned into the tetrapod hind limb.' "

The three authors drafted the paper eight months before Harvard's Jenkins died of cancer in 2012, but according to the Globe, the late paleontologist, considered a pioneer in his field, "as always, [insisted] that the first step to presenting their find to the wider scientific community was to assemble the visuals that would tell the story."